The present invention relates to a master cylinder for a hydraulic brake system of an automotive vehicle. Such master cylinders have included a housing, formed with a bore of which at least one piston is axially slidably guided in a sealed manner, the piston defining a pressure chamber in the master cylinder housing. At least one connection to an unpressurized supply reservoir is provided, comprising a central valve which controls the pressure fluid flow between a supply reservoir and the pressure chamber in dependence on the piston position. The central valve is composed of a sealing seat integrated in the piston, a valve pin penetrating a central bore of the piston and cooperating with a stop on the housing, a valve spring mounted on the piston and urging a valve member in the direction of the sealing seat. The valve member has been known to be made of elastomeric material and anchored in a groove in the valve pin.
Master cylinders with elastomeric central valves of this type are known in practice and described in many patent specifications (see for instance DE 38 35 396 A1 or DE 37 15 209 A1).
However, the valve member in the known arrangements extends radially inwardly over the valve pin in the area of the sealing seat. Admittedly, secure retention of the valve member on the valve pin together with the groove on the valve pin is known from DE 37 15 209 A1, but this construction has the shortcoming that the valve member is drawn into the central bore when the central valve is opened mechanically in opposition to a residual pressure in the pressure chamber. This can cause damage to the sealing lip of the valve member and failure of the brake circuit after relatively few cycles.
To remedy this shortcoming it is known to use steel ball central valves, but these are very susceptible to dirt. Another approach is to design the rubber part of the central valve as a sealing seat in the piston. The latter design involves increased structural complexity.
Therefore, the present invention the object to provide a master cylinder whose central valve can be opened in opposition to a residual pressure of roughly 80 bar (at present roughly 50 bar are usual) without damage, which is straightforward in its design and allows low-cost manufacture.